From a noise analysis point of view, many pressures sensors involving a capacitive sensing element, such as capacitor microphones, ultimately conform to the equivalent circuit shown in FIG. 1. Here e.sub.n is the series noise spectral density of the amplifier 100, commonly expressed in nV/.sqroot.Hz. This can be transformed into an equivalent "series noise resistance" R.sub.s using the Nyquist formula, ##EQU1## where K is Boltzmann's constant and T is the absolute temperature.
A second noise source is characterized by the shunt or "parallel" noise resistance R.sub.p. This corresponds not only to any actual parallel resistances (e.g., gate bias resistors for the amplifier input stage), but also to any system parallel leakage currents. Such currents can arise as FET gate leakage currents or bipolar transistor base currents, etc. The connection between such currents I and their equivalent resistance value R is given by the "fifty millivolt rule", ##EQU2## where q is the charge on the electron. (The value of 2KT/q is 50 mV at room temperature).
The important consideration that is clear from FIG. 1 is that the series noise from R.sub.s is dominant at high frequencies, while the parallel noise from R.sub.p dominates increasingly at low frequencies. The crossover between these two situations is characterized by T.sub.c the "noise corner time constant", ##EQU3## The significance of T.sub.c is that it is the reciprocal of the crossover frequency (in radians per second) at which the series and parallel noise contributions are equal. For applications involving relatively low frequencies, of which an audio microphone is an example, the parallel noise dominates the series noise. At frequencies much greater than the crossover frequency, the series noise dominates the parallel noise.
Pressure sensing devices such as capacitor microphones operate by generating a voltage .DELTA.V in response to the movement of the capacitive element. This signal voltage .DELTA.V is then to be compared with the noise voltages generated by R.sub.p and R.sub.s as they enter into the circuit of FIG. 1. This defines the signal to noise ratio (S/N), which in turn controls the sensitivity of the measurement, i.e. the lowest pressure change that can be detected. An alternative approach uses a known resonant phase shift method for measuring the small change in a capacitor value caused by the movement of one of its electrodes. In this method the capacitor of interest (e.g. a capacitor microphone) is used as the tuning capacitor in an LC resonant tuned circuit. Signal information is derived from the phase shift in tuned circuit response that results from any small changes .DELTA.C in the capacitor value. It can then be shown that for high quality factor (Q) tuned circuits, and large (S/N) ratios, the sensitivity can be extremely high. The advantage of this method is that there is no noise source equivalent to the parallel source R.sub.p. Sensors of this general type have been known for many years. However, the method suffers from two closely related problems in attempting to achieve the highest sensitivity. The first of these difficulties is that the AC drive voltage appearing across the capacitor itself produces an electrostatic force between its plates, which can produce mechanical instability. Secondly, particularly as the pressure sensitive plate of the capacitor is made lighter, the capacitance can be subject to large and uncontrolled drifts with temperature and time.